1. Field of the Invention
This invention relates to equipment and methods for separating a mixture of two liquids which are substantially mutually isoluble. If one or both of the liquids contain suspended solids, the solids can be removed or concentrated with the apparatus and method of the invention.
2. Description of the Prior Art
Mutually insoluble liquids are usually brought into direct contact with each other for any of three principal purposes, namely, transfer of dissolved material or heat from one liquid to the other, or to cause a chemical reaction between the two liquids. The most important of these purposes, and the one with which this invention is primarily concerned, is solvent extraction, i.e., the separation of solution components by causing their unequal distribution between two mutually insoluble liquids. As used herein, the phrase "mutually soluble liquids" means two liquids of such limited solubility in each other that for practical purposes they are considered mutually insoluble. Water and oil or kerosene are examples of two such liquids.
Solvent extraction processes, particularly in the mining industry, typically use multiple stages of mixer-settler units, with an aqueous phase and a water-insoluble organic phase, such as, kerosene, being repeatedly mixed and separated. The two phases flow countercurrently between stages of mixing tanks and settling tanks. One common version of solvent extraction apparatus and process is shown in U.S. Pat. No. 3,206,288. Such prior art equipment has had to be relatively large to be commercially feasible. This invention makes it possible to reduce substantially the size of solvent extraction apparatus, or conversely, to increase substantially the throughput of material being treated without increasing equipment size.
Separation of two immiscible liquids in a gravity settler tank involves many steps of which only two take a significant time. They are the aproach of a droplet toward the interface between the upper and lower layers, the drainage of the film of the continuous phase formed between the droplet and its bulk phase, and eventual rupture of the droplet, resulting in coalescence of the droplet with its bulk phase.
Where the two liquids are of appreciable volume and must be substantially fully separated, as in solvent extraction, the approach of a droplet toward the interface is of relatively minor importance, and coalescence of the droplet into its bulk phase is controlling. For example, in the solvent extraction of dissolved copper from leach liquid using a reagent dissolved in kerosene, the approach of a droplet toward the interface is typically accomplished in no more than about 10% of the flow path through the settler tank. For the remaining 90% of the length of the flow path, a dispersion band of almost uniform thickness is formed consisting of uncoalesced droplets dispersed in a continuous phase of the other liquid.
For any given liquid-liquid solvent extraction system, the settler tank size is established by the flow rate per unit of horizontal area of the settler, which corresponds to the desired dispersion band thickness at the outlet end of the settler tank. Increasing the thickness of the dispersion band increases the coalescence rate, or reduces the required area of the settler tank.
However, the thickness or height of the dispersion band has a practical economic limit. For a given settler depth, a thicker dispersion band means thinner clear liquid layers above and below the band. This causes a greater rish, when removing the clear liquids, of entraining some of the valuable components in the dispersion band material. Alternately, if the settler depth is increased to permit increasing the band thickness, the increased inventory of valuable liquids more than offsets the saving in settler tank area. As a result, practical considerations require that in a settler with a liquid depth of, say, 20 inches, the designed dispersion band thickness should be no more than about 2 inches at normal operations, or about 3 inches in upset or unstable conditions of operation. With this invention, the effective dispersion band thickness is made greater than that which is obtained in conventional equipment, resulting in faster coalescence with a concomitant reduction in equipment size, or increased throughput, or both.
The invention is also useful in removing suspended solids from liquids, such as in the treatment of crushed ores in mining operations. The typical process of mining breaks up excavated material into pieces ranging in size from the largest conveniently handled down to fine dust. Most operations require further crushing of the large material, generating further fine solids or dust.
When leaching is used to treat the crushed ore, the valuable constituent is dissolved out of the ore with a suitable liquid reagent (for example, an aqueous solution of a mineral acid) by adding the liquid to the ore, either as a spray, by percolating through a quiescent mass, or by agitating in a slurry. The valuable solution (called "leach liquor") drawn off contains suspended particles of solids, often further reduced in size and increased in number by the action of the leaching reagent. The quantity and size of these suspended particles are a function of the ore type, degree of crushing and exact measure of the leaching.
The suspended solids typically interfere with further processing of the leach liquor, and the custom has been to try to separate them out in large settling ponds or clarifiers, or to filter the liquor. Either settling or filtering requires large capital expenditures. The required clarification or filtration investment is often reduced by adding flocculents, a class of reagents which coagulates the suspended solids into larger clusters, which settle or filter more readily. The use of flocculents typically adds significantly to operating costs.
Leach liquors are often further processed by solvent extraction. As indicated above, in this process an organic chemical, with the capability of combining with a particular valuable constituent in the leach liquor, is dissolved in a suitable organic liquid, say, kerosene. When thoroughly mixed with the leach liquor, and then separated by gravity, the reagent-kerosene mixture absorbs the desired component from the aqueous leach liquor. The kerosene is then further processed to extract the component in a pure, relatively concentrated form for further operations, such as electrowinning.
The presence of suspended solids in the leach liquor is generally regarded as unacceptable for solvent extraction, not because of any particular difficulty in mixing with the reagent-kerosene mixture, but because the solids, in the subsequent separating stage, tend to accumulate at the interface between the aqueous leach liquor and the kerosene, interfering with coalescence and slowing the rate of separation to a point where normal operations must be suspended while the settler is cleaned. Pre-treatment of the aqueous leach liquor with a coagulant is not feasible because its residue produces an emulsion not separable in the settler tank.
This invention removes the solids in a liquid-liquid gravity settler tank, thereby eliminating the need for flocculants, filtration, or clarification.